1. Statement of the Technical Field
The invention concerns gain control in wideband distributed amplifiers. More specifically, the invention relates to an integrated high isolation bypass circuit suitable for use in monolithic microwave integrated circuits.
2. Description of the Related Art
Wideband distributed amplifiers have seen substantial use in high bandwidth electronic communications. Distributed amplifiers are circuit designs that leverage the impedances of a transmission line coupled with the intrinsic impedances of active field effect transistor (FET) cells to obtain a larger gain bandwidth than is possible using conventional amplifiers. As an input signal propagates along an input transmission line, active amplifier cells that connect the input line to the output induce an amplified, complimentary signal in the output transmission line. In an ideal distributed amplifier the gain of the amplified signal is determined by the transconductance and bias point of a single active FET cell, but also has a linear dependence on the number of active FET cells in the distributed amplifier. In practical circuits, this is limited by parasitic reactances and resistances of the FET cells and attached circuitry.
Systems with a wide amplitude range of input signals, i.e. distributed amplifiers, generally require some sort of gain control to maintain system linearity and sensitivity. Traditional solutions have relied on additional attenuation circuitry or external bypass switches that produce significant size, weight, and power costs. For example, adding a variable attenuator to the amplifier circuit is a typical solution to provide gain control. Alternatively, adding a bypass circuit to the amplifier will also provide gain control. However, both of these solutions require an additional component external to the amplifier that adds to chip area and contributes to other size, weight, and power costs. Additionally, the amplifier circuit is still receiving power when the output signal is externally attenuated. This leads to power inefficiencies. Linearity also suffers in the case of the external attenuator, as the non-linear amplifier component is still in the signal path.